ULTIMOBRANCHIAL OR POSTBRANCHIAL BODIES 165 



It is now generally believed that the entodermal epithelium of the thymus is converted 

 into reticular tissue and thymic corpuscles. The latter are the atrophic and hyalinized 

 remains of embryonic tubules and cords (Marine, 1915). The lymphoid cells were regarded 

 by Stohr as entodermal in origin, but most observers derive them from the mesoderm. 



Jugular Carotid Carotid 



vein artery artery 



Jugular vein 



Thyraud \-\%\ A Mi Fi Thyreoid 



Parathyreoid IV. ~ V\^L i^f^f Parathyreoid IV. 



Parathyroid III. Vf5^P7/I/ Parathyreoid III. 



Thymus - 



Superior vena cava 



FIG. 1 70. Reconstruction of the thymus, thyreoid and parathyreoid glands in a 26 mm. human 

 embryo (after Tourneaux and Verdun). X 15. 



THE PARATHYREOID GLANDS 



The dorsal diverticula of the third and fourth pharyngeal pouches 

 each give rise to a small mass of epithelial cells termed a parathyreoid 

 gland (Fig. 169). Two pairs of these bodies are thus formed, and, with 

 the atrophy of the ducts of the pharyngeal pouches, they are set free and 

 migrate caudal ward. They eventually lodge in the dorsal surface of the 

 thyreoid gland, the pair from the third pouch lying one on each side at 

 the caudal border of the thyreoid in line with the thymus anlages (Fig. 1 70). 

 The pair of parathyreoids derived from the fourth pouches are located 

 on each side near the cranial border of the thyreoid. Their solid bodies 

 are broken up into masses and cords of polyhedral entodermal cells 

 intermingled with blood vessels. In postfetal life, lumina may appear in 

 the cell masses and fill with a colloid-like secretion. 



THE ULTIMOBRANCHIAL OR POSTBRANCHIAL BODIES 



The ultimobranchial body is the derivative of the fifth pharyngeal 

 pouch (Fig. 169). By the atrophy of the ducts of the fourth pouches 

 they are set fra and migrate caudad with the parathyreoids. Each 

 forms a hollow^esicle which has been erroneously termed the lateral 

 thyreoid. According to Grosser and Verdun, it takes no part in forming 

 thyreoid tissue, but atrophies. Kingsbury (1915) denies the origin of 

 the ultimobranchial body from any specific pouch, and asserts it is "merely 

 formed by a continued growth activity in the branchial entoderm." 



